Critical properties of the spin Peierls transition in CuGeO₃

The spin-Peierls transition in CuGeO₃ has been studied using x-ray diffraction techniques. The order parameter of the (1/2,5,1/2) superlattice reflection has been measured with a temperature stability of ±5 mK. Careful analysis has been performed fitting the order parameter to both an ordinary power law in the reduced temperature as well as to a power law with the first correction to scaling term included in an attempt to determine a consistent value of the critical exponent β. We obtain a value of β of 0.36±0.03 at a transition temperature of 14.06±0.02 K. This value of β is consistent with conventional three-dimensional Heisenberg or XY behavior and clearly inconsistent with the tricritical values reported previously.